1. Field of the Invention
The present invention generally relates to hypodermic syringes used to inject medications and/or vaccines into the body of a human or animal. More specifically, this invention relates to a hypodermic syringe which is self-sheathing, such that the hypodermic needle is automatically and irreversibly retracted into a sheath after the contents of the syringe are dispensed, so as to prevent the needle from accidentally scratching or cutting the user or others, as well as to prevent the reuse of the syringe.
2. Description of the Prior Art
Hypodermic syringes are well known for their use in the dispensing of medication and vaccines for various types of medical treatment. Hypodermic syringes generally include a syringe body or barrel, a hypodermic needle secured to one end of the syringe barrel, and a plunger reciprocably received in the opposite end of the syringe barrel. The hypodermic needle is a hollow tube with a sharp point which enables the needle to penetrate the skin and muscle of the subject.
After its use, a hypodermic syringe poses a hazard unless and until the hypodermic needle has been removed or covered to prevent accidentally scratching or cutting the user or others. This hazard is particularly serious because of the risk of accidentally communicating a highly infectious disease, such as hepatitis B or acquired immune deficiency syndrome. Though health care workers are particularly at risk in that they are regularly exposed to the hazards posed by used hypodermic syringes, the misuse of discarded hypodermic syringes by intravenous drug users is also a notable problem, particularly in the spread of acquired immune deficiency syndrome.
Consequently, numerous types of hypodermic syringes which can be readily disabled have been suggested by the prior art. Generally, each attempts to retract the hypodermic needle into a sheath or into the barrel of the syringe, or provide a shield which can be extended to cover the needle after the syringe has been used, The prior art syringes differ primarily in the type of mechanism by which the shield is deployed and the manner in which the needle is prevented from being re-exposed. As an example, U.S. Pat. No. 5,135,510 to Maszkiewicz et al. discloses a syringe whose needle can be retracted into a protective sheath by operating a trigger which disengages a ratchet mechanism. Another example of a manually-operated retract mechanism is taught by U.S. Pat. No. 5,066,281 to Stevenson-Michener, wherein the needle is retracted after use by pulling back on the syringe barrel until the needle is drawn into a sheath, and the plunger is then rotated to permanently lock the barrel and plunger, such that the needle cannot be re-extended outside of the sheath. A further example is a cam-operated locking mechanism taught by U.S. Pat. No. 5,088,988 to Talonn et al., which makes possible a relatively small syringe suitable for use by dentists.
However, a disadvantage which the above nonreusable syringes have in common is that the user must be relied upon to disable the syringe after its use in order for the safety feature to be activated. Also, the ratchet mechanism taught by Maszkiewicz et al. can be readily defeated by disengaging the mating members of the ratchet mechanism. A disadvantage with the syringe taught by Stevenson-Michener is that visual observation of the syringe does not indicate whether the syringe has been disabled. In addition, a disadvantage with the syringe taught by Talonn et al. is the complexity of a locking collar which serves to disable the syringe, making the syringe more difficult to manufacture.
Other examples of nonreusable syringes taught by the prior art include U.S. Pat. No. 5,120,310 to Shaw, which relies on the elastic property of the syringe barrel to serve as a locking and release mechanism for the hypodermic needle. When released, the hypodermic needle is automatically retracted into the syringe barrel to provide the disabling function for the syringe. Though the syringe taught by Shaw has an advantage in that it is automatically disabled after all the fluid has been dispensed from the syringe, its operation relies on the user generating a force sufficient to overcome the gripping strength of the syringe, a characteristic based on properties of the syringe which may be difficult to control in mass production. In addition, the syringe taught by Shaw requires additional components disposed within the syringe barrel, which reduces the relative capacity of the syringe for a given size, as well as contributes additional sources for contamination of the contents of the syringe. Furthermore, the entire contents must be dispensed from the syringe taught by Shaw before the locking mechanism is activated.
In contrast, U.S. Pat. No. 5,026,346 to Spanner et al. employs a disabling device which is automatic and does not require dispensing the entire contents of the syringe barrel to activate the disabling mechanism. However, Spanner et al. involves a rather complicated construction which operates to break the seal on the plunger, so as to prevent subsequent refilling of the syringe.
While the above examples represent only a fraction of the syringes taught by the prior art, they serve to highlight some of the more common disadvantages associated with the use of known nonreusable syringes. Generally, from the above discussion, it can be readily appreciated that the prior art does not teach within a single disclosure a nonreusable syringe which 1) does not rely upon the user to manually disable the syringe after its use; 2) does not require a complicated locking mechanism; 3) does not rely on a locking mechanism which can be easily defeated; 4) does not require the entire contents of the syringe to be used before the syringe can be disabled; 5) does not require close inspection to determine whether the syringe has been disabled; and 6) does not significantly reduce the fluid capacity of the syringe.
Accordingly, what is needed is a cost-efficient, readily manufacturable syringe assembly which employs an uncomplicated locking mechanism that visibly renders the syringe unusable, wherein the locking mechanism is activated prior to the use of the syringe, such that any subsequent use of the syringe results in the permanent disablement of the syringe.